Update:
I haven't removed sdb yet. Looking at the error I'm getting, READ_FPDMA_QUEUED could well indicate a driver or controller issue rather than a failed drive. Indeed, smartctl doesn't seem to indicate a drive issue that I can see (although I admit to finding smartctl output very hard to interpret): http://paste.ubuntu.com/5710000/
I'm going to try replacing cables etc and seeing if that makes a difference before swapping the drive out, as I am as sure as I can be that sda is doing a damn good impression of a dodo right now (but worst case it's still available to work on if I need to).
Mark -- Mark Rogers // More Solutions Ltd (Peterborough Office) // 0844 251 1450 Registered in England (0456 0902) @ 13 Clarke Rd, Milton Keynes, MK1 1LG